Serial localization with indecision (SLI) has been used to reduce demodulation complexity while maintaining sufficient performance. For example, SLI was implemented with MLD (Maximum Likelihood Detector) in commonly assigned U.S. patent application Ser. No. 12/549,132, entitled “DEMODULATION USING SERIAL LOCALIZATION WITH INDECISION” and filed 27 Aug. 2009, which is incorporated herein by reference in its entirety. SLI was also implemented with MLSE (Maximum Likelihood Sequence Estimator) in commonly assigned U.S. patent application Ser. No. 12/549,143, entitled “EQUALIZATION USING SERIAL LOCALIZATION WITH INDECISION” and filed 27 Aug. 2009, which is incorporated herein by reference in its entirety. SLI was implemented as part of an MSA (Multi-Stage Arbitration) demodulator in commonly assigned U.S. patent application Ser. No. 12/549,157, entitled “JOINT DEMODULATION AND INTERFERENCE SUPPRESSION USING SERIAL LOCALIZATION” and filed 27 Aug. 2009, which is incorporated herein by reference in its entirety.
SLI has an indecision feature which arises from representing the modulation constellation with overlapping subsets of centroid-based values. The indecision feature is beneficial for multi-stage structures, because the indecision reduces irreversible erroneous decisions in earlier stages. A particular SLI block represents subsets of constellation symbols by their respective centroid or other centroid-based value, and treats the centroid-based values as a demodulation constellation. By detecting a particular centroid-based value, an SLI block effectively localizes the search for the final symbol decision, since the next stage focuses on the symbol subset associated with the detected centroid-based value.
SLI becomes more beneficial where the effective constellation is very large. Larger signal constellations such as 16, 32 and 64 QAM (Quadrature Amplitude Modulation) have been adopted in EDGE (Enhanced Data rates for GSM Evolution), HSPA (High Speed Packet Access), LTE (Long Term Evolution) and WiMax (Worldwide Interoperability for Microwave Access). In HSPA, multi-code transmission creates even larger effective constellations. MIMO (Multiple-Input, Multiple-Output) schemes have been adopted in HSPA, LTE and WiMax, creating large effective constellations. Demodulation complexity is further exacerbated when these techniques occur in combination. Another issue is ISI (Inter-Symbol Interference), which makes the demodulation complexity grow with a power of the size of the effective constellation.